The invention relates to a method for the continuous melting of inorganic salts and to a furnace installation for realizing the method.
Melted inorganic salts of the type under discussion are used in different industrial plants as a heat-storage medium or as a heat-transfer medium. The melted inorganic salts are used in particular for storing heat in solar power plants or other thermal power plants. They are designed to store excess amounts of heat from ventilation systems, as well as for the transfer of heat (i.e., heating) or the dissipation of heat (i.e., cooling) in industrial-size chemical reactors.
As compared to organic heat storage or heat transfer media, melted inorganic salts have the advantage of having an extremely high thermal capacity (specific heat) and can be used at high temperatures since they have a low tendency to decompose and a high evaporation point. Inorganic salts are furthermore cheap and available worldwide.
Melted inorganic salts can be used as thermal storage media for the process of generating electrical energy in solar power plants, which has seen a considerable increase in recent years [Bielefelder Verlag GmbH & Co. KG, “Sonne, Wind und Wärme (Sun, Wind and Energy)” [online], 11 (2004) 54-56, [retrieved on Feb. 21, 2008] Retrieved from http://www.sunwindenergy.com/swe/content/archiv/archiv.ph p?showall=1; FLAGSOL GmbH 2008, “Andasol Projects” [online], [retrieved on Feb. 21, 2008] Retrieved from http://www.flagsol.com/andasol_projects.htm; Solar Millenim AG [online], [retrieved on Feb. 21, 2008] Retrieved from http://www.solarmillenium.de].
The storage of energy produced with solar technology represents a special challenge because the production of this electrical energy must also be ensured during the night.
Liquefied mixtures of potassium nitrate (KNO3) and sodium nitrate (NaNO3) at a ratio of 40/60% by weight (melting point approximately 250° C.) can be used for this purpose, wherein this liquid mixture is stored in extremely large, thermally insulated storage containers. During the day, the melted salt mixture is heated to a high temperature T1, for example to 400° C., with the aid of concentrated energy from the sun and is stored inside a hot tank. During the night, the hot salt melt is removed from the hot tank and is conducted over a heat-exchanger system, wherein a portion of the stored energy is used for producing electricity with the aid of steam turbines. In the process, the salt melt cools down again to a temperature T2, for example 290° C., and is then stored while still in the liquid form in a cold tank, approximately at a temperature of 290° C. During the following day, the salt melt is again heated up to the high temperature of 400° C. by a portion of the irradiating energy from the sun, whereupon it is again transferred to the hot tank and the cycle can repeat itself.
The storage tanks of industrial solar power plants, which have a capacity of approximately 50 megawatt and can hold approximately 30,000 tons of the melted storage salt, have dimensions in the order of 30 m in diameter and 15 to 20 m in height.
The salt or the salt mixture must be heated at least to the temperature of the cold tank for the operational start-up of such a heat-storage system.
In concrete terms, it means that for the start-up of a solar power plant with approximately 50 megawatt capacity, an amount of 30,000 tons of the aforementioned potassium/sodium nitrate mixture must be heated to 290°-300° C. and stored inside an insulated storage tank.
Traditional melting furnaces or heaters inside the container cannot be used for melting such large amounts of inorganic salts or salt mixtures in the crystalline form of the crude material. The attempt to melt such large amounts of salt inside a container would only result in a localized melting because of the poor heat conduction of the crystalline crude materials. Otherwise it would take an unrealistically longtime until the total salt amount would be melted.